There exist commercially today, basically two processes for the manufacture of titanium dioxide pigment. The first is the chlorination process whereby titanium ore is reacted with a carbon source and chlorine to form titanium tetrachloride which, in turn, is oxidized to form the titanium dioxide and recyclable chlorine. The second process is the sulfuric acid process whereby titanium ore is digested with sulfuric acid to form a cake containing the sulfate salts of the iron, titanium and other metals contained in the ore. Impurities are then removed and the acid content is adjusted to that desired. The digestion liquor is then hydrolyzed to form hydrous titanium dioxide which, upon calcination, converts to either anhydrous anatase or rutile titanium dioxide pigment.
A large and still increasing proportion of all titanium dioxide pigment used in paper and paint manufacture is marketed and used in a slurry form. Because of the high cost of transportation, it is becoming increasingly important to ship said slurries at their highest practical solids concentration. However, the requirements for commercial slurries are that the slurry viscosity be low enough for pumping and not change appreciably upon aging. Although slurry viscosity may be controlled somewhat by proper dispersant addition, the presence of sulfate ions chemically bound to the TiO.sub.2 in te slurry has a tendency to cause flocculation and thereby increase the viscosity of the slurry. If a method could be found which would prevent or minimize the tendency of such slurries to increase in viscosity over their lifetimes, such method would satisfy long-felt need in the industry.
It has now been found that the viscosity of high solids pigment slurries can be reduced by subjecting the slurry to electrodialysis. The use of electrodialysis obviates the necessity for the lengthy and expensive reprocessing which accompanies many alternative viscosity reducing procedures contemplated or used commercially. The reduction of the viscosity of high solids pigment slurries in accordance with the present invention enables the manufacturer to produce and ship higher solids content slurries having substantially stable viscosities equivalent to or lower than previous commercial slurries at an ultimate economic advantage.